A scanning electron microscope (SEM) is a type of electron microscope that images a sample surface by scanning it with a high-energy beam of electrons in a raster scan pattern. As the scanning electron beam contacts the surface of a sample, backscattered and/or secondary electrons are emitted from the sample surface. Semiconductor inspection, analysis and metrology may be performed by detecting these secondary electrons. A point-by-point visual representation of the sample may be obtained on a cathode ray tube (CRT) screen or other display device as the electron beam controllably scans the sample.
SEMs operate by creating a beam of electrons accelerated to energies of up to several thousand electron volts. The electron beam may be focused to a small diameter and scanned across a feature of interest in the scanned sample. When the electron beam strikes the surface of the sample, low energy secondary electrons are emitted. The yield of secondary electrons depends on various factors including the work function of the material of the sample, the topography of the sample, the curvature of the sample surface, and the like. These secondary electrons can be employed to distinguish between different materials on a sample surface since different materials may have significantly different work functions. When multiple SEMs are used to analyze the same sample, a need exists for calibrating each of the SEMs with respect to one another.